1. Field of the Invention
The present invention pertains to the field of communications and, in particular, to combining diverse antenna signals.
2. Description of Related Art
Signals propagating in a near earth environment are subject to fading because of multipath effects, obstructions, antenna nulls, and other effects. One known technique for mitigating fading effects is antenna diversity, also called space diversity. This known technique takes advantage of the fact that signal fading is generally not the same at all locations; rather, signal fading may vary rapidly at a given location due to changes in relative position of a transmitter with respect to a receiving antenna or antennas (some of which may be mobile), interference, signal echoing, and other foreseeable and unforeseeable factors. In some cases, for example, the quality of a received signal at a given antenna location may vary over periods as short as ten milliseconds or less, depending on factors such as vehicle speed (when communication is with a mobile unit.
To mitigate the effects of fading, multiple antennas are used to collect energy at physically separated locations. Subsequent processing may then attempt to select a single antenna providing the best signal/noise ratio, on a dynamic basis. It is also known to combine energy from the multiplicity of antennas to attempt to maximize the signal/noise ratio.
Because antenna diversity relies on the fact that antennas are located in different physical locations, the antennas may be physically separated from each other and from the receiver by large distances. When the antennas are physically separated from the receiver electronics, conventional practice is to run separate cables coupling each antenna to the receiver, called xe2x80x9cbackhaul cablesxe2x80x9d. While this conventional technique may achieve the purpose of coupling the receiver to all of the antennas, it is subject to the drawback that when a physical separation between the antennas and the receiver is large, running multiple backhaul cables can be costly. For example, it is not unusual for an antenna to be separated from the receiver by as much as five miles. Using multiple backhaul cables to service multiple antennas over such distances may entail substantial expense.
The need for multiple backhaul cables to transmit the signal bandwidths required by multiple antennas has further drawbacks as well. For example, there is presently a desire to utilize the existing cable TV infrastructure to provide various telecommunication and other multimedia services. Thus, backhaul cables associated with existing cable TV systems may be utilized to carry a variety of telecommunications data including, for example, television, video, cellular, fax, voice communication, and other types of data. In many cases, however, the backhaul capabilities of conventional cable TV systems, which often have only a single backhaul cable available, are insufficient to handle the bandwidth required to support antenna diversity for cellular systems or other telecommunication systems.
It would therefore be advantageous to provide a technique that minimizes the number of backhaul cables needed to couple a plurality of antennas to a receiver. It would further be advantageous to provide a technique for increasing the amount of data that may be transmitted over existing backhaul cables having limited bandwidth capabilities.
The invention provides in one aspect a technique for antenna diversity minimizing the number of backhaul cables needed for a plurality of antennas. In one embodiment, a single cable couples the receiver to a plurality of antennas, and the signals from the antennas are combined onto the single cable. The technique for combining onto a single backhaul cable may employ frequency offsets, spread spectrum code division multiplexing, and/or time division multiplexing. At the receiver, the signals from the antennas are decoupled or otherwise separated. In the case of frequency offsets, the antenna signals are decoupled by splitting the backhaul signal into a plurality of duplicate signals, frequency shifting selected ones of the duplicate signals, and correlating said frequency shifted signals. In the case of spread spectrum code division, the antenna signals are decoupled by splitting the backhaul signal into a plurality of duplicate signals and demultiplexing each of the duplicate signals with a different spread spectrum code. One or more antennas may be selected for communication in response decoupling the antenna signals.
In a preferred embodiment, the signals from the antennas are multiplexed and demultiplexed using a spread spectrum communication technique, with isolation between the different signals being provided by the use of different spread spectrum code sequences. Antenna diversity can thus be supported without substantially increasing bandwidth requirements for the backhaul cable. In one such embodiment, a spread spectrum bandwidth for the multiplexed signals is about 10 MHz, and a frequency separation between the multiplexed signals is about 100 KHz.